Gallery: Add items to GalleryDataModel before loading their metadata.

ui/file_manager/gallery/js/gallery_data_model_unittest.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 
 var model;
 var fileSystem;
 var item;
 
+/**
+ * Mock thumbnail model.
+ */
+function ThumbnailModel() {
+}
+
+ThumbnailModel.prototype.get = function(entries) {
+  return Promise.resolve(entries.map(function() {
+    return {};
+  }));
+};
+
 function setUp() {
   model = new GalleryDataModel(
-      /* Mock MetadataModel */{},
+      /* Mock MetadataModel */{
+        get: function() {
+          return Promise.resolve([{}]);
+        }
+      },
       /* Mock EntryListWatcher */{});
   fileSystem = new MockFileSystem('volumeId');
   item = new Gallery.Item(
       new MockEntry(fileSystem, '/test.jpg'),
       null,
       /* metadataItem */ {},
       /* thumbnailMetadataItem */ {},
       /* original */ true);
 }
 
@@ skipping 29 matching lines @@
     // Gallery item track new file.
     this.entry_ = new MockEntry(fileSystem, '/test (1).jpg');
     callback(true);
   };
   model.push(item);
   reportPromise(
       model.saveItem({}, item, document.createElement('canvas'), false).
           then(function() { assertEquals(2, model.length); }),
       callback);
 }